...comparing apples to rocks here. Let's see...the objective of NT textual criticism is to improve the version of the NT to be closer to the original manuscript and thus a better reflection of the version inspired by God. NT textual criticism uses the techniques of rational man. Who knows, they may be inspired!
Joseph Smith, a prophet of God, worked at improving the NT text to be closer to the original manuscript and thus a better reflection of the version inspired by God. As you pointed out, he was not using the techniques of NT textual criticism, nor did he have any other manuscripts to draw from. He was doing the work strictly based on inspiration from God.
Which technique do you think will be more successful? So you see, I agree that the comparison is apples to rocks. I prefer the inspiration of a prophet of God over the rational man any day. :-)
As you pointed out, he was not using the techniques of NT textual criticism, nor did he have any other manuscripts to draw from. He was doing the work strictly based on inspiration from God. And side by side, smith's 'translation' is bogus with typical signature of one who hasn't a clue with what he is dealing with. Example - Psalm 95:11 is quoted three times in Hebrews. The JST adds nothing to the first quotation (Heb. 3:11), adds 22 words to the second quotation (4:3), and adds 5 words to the third quotation just two verses later (4:5). Apparently, smith didn’t understand that Hebrews was quoting the same Old Testament text in all three verses.
Take also this
John 4:2 — (Though Jesus himself baptized not, but his disciples,) KJV
John 4:3 — Now the Lord knew this, though he himself baptized not so many as his disciples; JST
John 4:2 — 
So if smith was so 'inspired' why didn't he translate to the more correct MS?
Smith's translation skills are bogus - he went about trying to force his doctrine upon the bible - regardless of the 'correct'. No, I trust God in the preservation of His word - and not that of a convicted treasure seeker.